Homeowners will be allowed to build extensions and carry out major renovations without the cost or delay of seeking planning approval under temporary rules intended to help the construction industry.

David Cameron and Nick Clegg will today stage a show of Coalition unity as they attempt to give new impetus to economic growth by streamlining planning and encouraging building. The centrepiece of the announcement will be a watering down of the planning rules covering changes to homes.

Almost 200,000 households apply each year for permission to make improvements such as conservatories, rear extensions and garage conversions. Application fees are typically £150 and professional fees can cost thousands.

Under the current rules, single-storey rear extensions can be constructed without planning permission as long as they do not extend beyond the rear wall of the original house by a set distance. For semi-detached properties, the limit is three metres and for detached homes four metres. Under temporary rules to be put in place next month, those limits will be doubled, meaning many extensions currently requiring permission could go ahead. The changes will not apply to loft extensions.

But the announcement has been overshadowed by concerns the Government may be compromising on its green agenda.

Liberal Democrats have reacted with dismay to the appointment of "anti-green" Tory ministers in the Department of Environment, Food and Rural Affairs and to the Department of Energy and Climate Change.

Party sources claim the appointments have exposed Mr Cameron's green credentials to be "skin deep" and vowed to ensure the Coalition delivers on its promises to tackle climate change and boost renewable energy.

Owen Paterson, the new Environment Secretary, is sceptical about global warming and is a strong critic of wind farms. Nick Boles, the new Planning minister, has previously advocated building on the green belt and John Hayes, who was appointed Energy minister, has opposed siting wind turbines in his constituency.

Charles Hendry, who was regarded as a keen advocate of renewable energy and has been replaced by Mr Hayes, was sacked from the Government. Justine Greening was moved out of her post as Transport Secretary as Mr Cameron reopens the question of whether a third runway should be built at Heathrow. She clashed on the issue with Chancellor George Osborne, who is pressing for an early decision on Heathrow.

A senior Liberal Democrat source said: "The Tory commitment to tackling climate change increasingly seems skin deep, but we will make sure the Government sticks to its promises in the Coalition agreement."

Ed Davey, the Liberal Democrat Climate Change Secretary, is taking direct responsibility for promoting renewable energy – a policy area that was previously left to Mr Hendry. Mr Cameron confirmed moves to take the political heat out of the future of Heathrow by setting up a cross-party commission to examine the issue of airport capacity in the South-east. It is expected to be headed by Sir Howard Davies, the former head of the Confederation of British Industry.

The move is likely to delay a final decision on whether to build an extra runway at Heathrow – as opposed to expanding Gatwick or Stansted or constructing a new airport in the Thames Estuary – until after the next election. Mr Cameron told MPs yesterday: "What we need to do is build a process that hopefully has cross-party support, so we can look carefully at this issue and deliver changes that will address the problems of capacity we will have in future years."

But the Mayor of London, Boris Johnson, stepped up the pressure on Mr Cameron by challenging him to rule out expanding Heathrow for the foreseeable future.

There is also disquiet within Conservative ranks over the influence that Mr Osborne had wielded over many ministerial appointments. One senior party source said Ms Greening's switch to become International Development Secretary was a "sign that people should not mess with George Osborne".

The first meeting of the reshaped Cabinet yesterday was dominated by the need for growth. Mr Cameron told his new team: "This is a huge effort for right across Government and it absolutely has got to have as much pace and effort and energy as we can possibly muster. It is the biggest challenge that we face in our country, dealing with these twin threats of deficit and low growth."

The Government will also announce new help for 16,500 aspiring homebuyers by expanding the FirstBuy scheme.

Has been quoted as saying that planning "can't work". A long-term Cameron ally, his appointment has "raised eyebrows" among defenders of the green belt, with whom the Government reached an uneasy truce over planning reforms in April. At the time, he described opponents of planning deregulation as "latter-day luddites" who deploy "hysterical scare-mongering".

Owen Paterson; Environment Minister

Has a record of backing policies that are anathema to environmentalists. Supports "fracking" for shale gas and is keen to expand the UK's airport capacity. Used his time as shadow Defra minister in 2005-06 to voraciously campaign for a badger cull and was also a vocal critic of the ban on fox hunting. Described by the Countryside Alliance as "a staunch supporter of all field sports".

John Hayes; Energy Minister

One of the Tories' most vociferous critics of wind farms. In 2009 he called turbines "a terrible intrusion" that failed the "twin tests of environmental and economic sustainability." He replaces the highly regarded energies-sector expert Charles Hendry. Mr Hayes is likely to take George Osborne's side in any dispute with Liberal Democrat Climate Change Secretary Ed Davey over the energy strategy.